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DYNAMICAL OCEANOGRAPHY
Summary
The El Ni˜ no phenomenon is associated with an interannual sea surface temperature anomaly in the eastern to central Pacific. There are
connected changes in the sea level pressure and surface winds which
are called the Southern Oscillation.
A positive SST anomaly in the eastern Pacific leads to low-level heating of the tropical atmosphere and to a westerly wind anomaly located
west of the SST anomaly and hence a weakening of the trade winds.
A weakening of the trade winds leads to a reduction of upwelling and
a reduction in the slope of the thermocline.
There exist three positive feedbacks in the coupled system: the thermocline feedback, the upwelling feedback and the zonal advection
feedback.
El Ni˜ no is brought about through coupled interactions between the
equatorial ocean and the global atmosphere. Anomalies in sea surface temperature induce wind-stress anomalies which cause changes
in thermocline depth and upwelling and the three feedbacks can amplify these anomalies. The ocean adjusts to the changes in wind stress
through the propagation of equatorial waves. The ocean basin adjustment provides a negative feedback which damps the anomalies and
which induces a transition to the opposite phase (La Ni˜ na). This is the
essence of the delayed oscillator view of ENSO.
DYNAMICAL OCEANOGRAPHY
Summary
The El Ni˜ no phenomenon is associated with an interannual sea surface temperature anomaly in the eastern to central Pacific. There are
connected changes in the sea level pressure and surface winds which
are called the Southern Oscillation.
A positive SST anomaly in the eastern Pacific leads to low-level heating of the tropical atmosphere and to a westerly wind anomaly located
west of the SST anomaly and hence a weakening of the trade winds.
A weakening of the trade winds leads to a reduction of upwelling and
a reduction in the slope of the thermocline.
There exist three positive feedbacks in the coupled system: the thermocline feedback, the upwelling feedback and the zonal advection
feedback.
El Ni˜ no is brought about through coupled interactions between the
equatorial ocean and the global atmosphere. Anomalies in sea surface temperature induce wind-stress anomalies which cause changes
in thermocline depth and upwelling and the three feedbacks can amplify these anomalies. The ocean adjusts to the changes in wind stress
through the propagation of equatorial waves. The ocean basin adjustment provides a negative feedback which damps the anomalies and
which induces a transition to the opposite phase (La Ni˜ na). This is the
essence of the delayed oscillator view of ENSO.